Method of making lithograph blankets



Oct. 10, 1950 w. H. SMITH mnon OF MAKING LITHOGRAPH BLANKETS 2 Sheets-Sheet 1 Filed Nov. 17, 1947 3 MZZeIHSIIziHz,

Alike 2412 W. H. SMITH 'ME'I'I-XODOF MAKING LITHOGRAPH BLANKETS 2 She ets-Sheet 2 Filed Nov. 17, 1947 Patented Oct. 10, 1950 METHOD OF MAKING LITHOGRAPH BLANKETS Walter H. .Smith, Toronto, Ontario, Canada; Mary Agnes Johnston, cxecutrix of Walter H. Smith, deceased, assignor to Commercial Lithograph Company, Lid.,

Canada Toronto, Ontario,

Application November 17, 1947, Serial No. 786,519

1 Claim. 1

This invention relates to lithograph blankets and to a method and means for making the blanket.

The present day lithograph blankets are all made of canvas-backed rubber sheeting. A The thickness of these blankets is not uniform and furthermore, all of, these blankets have a tendency to stretch so that :they lack uniformity in use even though their thickness may be reasonably uniform. In an effort to obtain uniformity in thicknessit has been proposed that their surfaces be ground down so that they will be even.

To perform this grinding operation the blanket is normally positioned in a plane and a grinding tool passed thereover to remove the high spots on the surface. This method has proved ineffective, however, due to the elasticity of the rubber as the excess rubber in the high spots has .a tendency to be pushed in front of the grinding tool and then snapped back beneath so that theresultant surface is uneven. Further, when a grinding wheel is employed, .the pull of the wheel forces the rubber ahead of it for a short distance so that an uneven surface is created, even if it were not uneven in the beginning. I

It is absolutely essential that' a lithograph blanket have a substantially uniformfthickness as this blanket is the actual printing member.

In the usual offset-lithograph press, a printing plate is wrapped around a printing cylinder, and

covered cylinder, using slightly less pressure during this operation than the required ultimate pressure in the regular printing operation. This results in only the high spots of the blanket becoming inked. The pressman then removes the blanket and with paste and thin paper, he pastes suificient layers of paper under the low spots to the rubber lithograph blanketis .wrapped around an impression cylinder which is juxtaposed relative to the printing cylinder, so as to transfer an image from the printingplate to the rubbersurface of the blanket which in turnofisets or prints the received image on a sheet ofpaper orother sheet material such as tin, foil, and the like.

There can be'no unnecessary pressure exerted between the blanket and either the printing plate or the paper sheet except for the mere pressure of contact that is necessary and which may be in the neighborhood of /1000 of an inch squeeze. As the entire surface of the blanket "must be. pressed into contact with the sheet to be printed, any thickness, as a high spot in the rubber surface, would obviously be subject to a greater pressure than is required for printing and consequent 1y must be compensated by building up thelow portions thereof.

At the present time when installing a" new blanket, it is first attached with any of the many well known methods of attaching, and stretched taut by the pressman. An impression is'pulled between the plate, which for this purpose is inked solid all over the printing surface, and the rubber thickness.

bring them up to the high spots. This is known as make ready and requires considerable "manual labor and time on a large press, as much as several hours when accurate colour work'is to be run. After several tests, all the low spots are built up so that there is an even inking of the blanket. Naturally, if there are no low spots this is eliminated.

Furthermore, the thickness of the prior art blankets occupy an undesirably large amount of space in being attached to certain types of impression cylinders, such as that disclosed in my Patent No. 1,949,132., so that the size of the actual printing surface is limited.

Having in mind the defects of the prior art lithograph blankets and methods for making such blankets, it is an. object of the presentinvention to provide an absolutely non-stretchable rubber surfaced blanket.

- 1 It is another objector the invention to provide a rubber surfaced lithograph blanket of uniform It is still another object of, the invention to provide a lithograph blanket having a greater printing area for the given size of the blanket in before noted.

on certain types of impression cylinders as here- It is a further object of the invention to provide a method for producing a rubbersurface lithograph blanket and for grinding the rubber sur face of the blanket to uniformity. It is a further object of the invention to provide means for mounting and holding a rubber surfaced blanket upon an arbor or cylinder. 7

The foregoing objects, and others ancillary thereto which will become apparent from the following description, are preferably accomplished by a lithograph blanket comprising a rubber sheet securedto a thin sheet metal backing so that the 'metal backing will preclude the possibility of stretching and willoccupy less space when .mounted on an impression cylinder so as to provide a greater printing area for a given size of blanket. Thisblanket may be ground to absolute uniform. thickness by mounting tightly around a'cylinder or arbor with the rubber facing outward. The arbor may ,then be mounted in any of the many grinding lathes or machines which can be used for grinding cylindrical surfaces. With the rubber held tightly on a cylindrical form it does not give or stretch in front of the grinding tool as is the case when it is flat. A device for mounting the blanket on the arbor may comprise a cylinder having means for gripping the ends of the metal backing, one of said means being movable relative to the other to pull the blanket tightly against the cylindrical surface. The arbor may be movably mounted in a standard including a friction roller for rubbing against the outer rubber surface of the blanket to pull it tightly on the cylinder as the cylinder is rotated. This friction roller may be movably mounted to and from the cylinder and have a clutch mechanism for adjusting the frictional resistance of the roller.

The novel features that are considered characteristic of the invention are set forth with particularity in the appended claim. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein like reference characters indicate like parts throughout and in which:

Fig. 1 is a view in perspective of lithograph blanket in accordance with the invention;

Fig. 2 is a cross-sectional view taken on line 22 of Fig. 1;

Fig. 3 is a cross-sectional view taken transversely through an impression cylinder having a lithograph blanket mounted thereon;

Fig. 4 is an end view and elevation of a device for mounting a blanket on a cylinder and pull ing it tautly there around; and

Fig. 5 is a front view and elevation of a device shown in Fig. 4.

Referring now to the drawings and specifically to Fig. 1. a lithograph blanket in accordance with th present invention comprises a rubber sheet ID that is vulcanized to a thin metal sheet II which has its end portions I2 extending beyond the ends of the rubber sheetIIl, at least one of said end portions I2, which forms, the front end, having registering holes I3 for'cooperation with registering pins on an impression cylinder or grinding mandrel, whereas the other end porinch thick. Whereas the rubber sheet I!) is of a thickness that will make the thickness of the entire blanket about equal or slightly thinner than the regular lithograph blanket. It is obvious that a blanket of this type is absolutely non-stretchable due to the metal backing and furthermore, it has the additional advantage of providing a greater printing area for a given size 'of the blanket due to the thinness of the metal backing.

' As an example of this latter feature, reference will be made to the dimensions of a standard size of lithograph press which comprises an impression cylinder I5, as shown in Fig. 3, which has the blanket wrapped therearound with the metal backing I I against the surface of the cylinder and;

cylinder with the blanket thereon has a total circumference of approximately 8 whereas the actual printing surface around the cylinder is approximately 8%". In using the regular prior art canvas-backed blanket, the slot I6 had to be in the neighborhood of /64 of an inch wide in order to receive the two ends of the blanket and a lock bar or wedge I 'I. This regular blanket is in the neighborhood of /1000 of an inch thick so that the two ends made a total of /1000 of an inch in all and the wedge or lock bar is in the neighborhood of A, of an inch thick making a total of /1000 of an inch as the minimum necessary width of the slot I6. In using a blanket of the present invention however the total thickness of the ends I2 and the bar I'I amounts to /1000 of an inch so that the slot I5 can be smaller and increase the printing circumference of the impression cylinder. Furthermore, the total thickness of the metal backing II' and rubber sheet II] need only be 40-45 thousandths of an inch thick so that the cylinder I5 may be 2.61 inches in diameter to bring the total diameter with the blanket thereon, up to 2.7 inches which is the diameter of the printing surface of the cylinder in this specific press. On the other hand, with the prior art blanket, which is 5 1000 of an inch in thickness, the cylinder had to be 2.57 inches and the opening or slot I6, .270 of an inch wide which reduced the effective printing area to less than 8 inches at the edge of the rubber when clamped in place. Whereas the new metal backed blanket being cut back on each edge /1000 of an inch to provide clearness of the bends in the metal at each end, the printing surface results in an area of 8.168 inches. Thus it will be seen that the metal backing not only provides a nonstretchable blanket but permits a thinner blanket occupying less of the available cylindrical surface in being clamped to the impression cylinder and providing a larger actual printing area.

In order to obtain a. blanket of absolutely uniform thickness it is necessary to grind the surface of the rubber sheet I0. Further, the grinding action of the surface creates a superior printing or transfer surface, for receiving and offsetting the image. I 7

It has been found that superior results in the grinding operation are obtained by mounting the blanket on an arbor and grinding it in a lathe rather than grinding itin a plane. When the blanket is pulled taut around a cylindrical-surface the rubber layer to the outside of the blanket is stretched slightly and positioned under substantially the same conditions as inuse whereas when flat the rubber is not stretched and has greater elasticity, so that it tends slightly to be forced ahead of the grinding wheel particularly in the thicker spots. Obviously with the prior art offset blankets which are canvas-backed, they are not true in the first place and further, it is impossible to grind them accurately regardless of their being mounted cylindrically or on the flat as the thicker layer of rubber permits greater deformation during the grinding operation and the canvas-backing permits stretching to a suflicient degree to throw the entire blanket off.

In forming the present blanket, however, it is cut to the required size both as to the rubber printing sheet In and'themetal backing II with the projecting ends I2. The front or leading edge would be cut to provide the registering edge which is used to locate the blanket so that the ends would be correctly formed. It is obvious that these ends can be alike,as shown in Fig. '1 or may belfiush withtherubberand usedlin th same manner as prior .art blankets although the projecting metal ends are preferred;

To grind the blanket,.it is mounted ona cylinder which is best shownin Fig.4. The cylinder 29 preferably comprises a cylindrical shell 2! mounted on a shaft 22 and-having a cutaway open side 23 to accommodate a transversely extending bar 24 supported at its ends by radially extending arms 25 which are pivotally mounted on the shaft 22, beyond the ends of the cylindrical mandrel. One edge portion 26 of the cylindrical surface of the mandrel 2! isbevelled to accommodate clamps 2T within the circumference of the mandrel. The clamps 2'! are for securing the front registering end !2 of the blanket to the cylinder and cooperate with pins 28, extending through the registering holes !3, for registering the front end of the metal backing sheet I The clam s 2'! are secured by screws 28'.

When the blanket is wrapped around the mandrel 2!, the opposite end portion [2 is located adjacent the other edge of the cutaway portion 23 and may be clamped to the transverse bar 24 by clamps 29 and screws 36. One or more bolts 3! are threaded through the bar 2 to abut the substantially radial edge 32 of the cutaway portion 23 of the cylinder 2!, so that, after the blanket end i2 is clamped to the transverse bar 2d, the bolt 3! may be screwed to bear against the bearing surface 32 to pull the blanket taut around the cylindrical mandrel 2!.

In order to expedite the mounting of the blanket on the cylinder 2! and secure sufficient tautness thereon, the cylinder is preferably pressure roller 44 will contact the blanket on the cylinder v2! regardless of any variations of thickness of the blanket between the sides thereof. The arms lfl terminate in extensions 55 having bearings 56 for receiving pins 5'! which pivotally support telescoping rods 56 that are pivoted by pins 5.9, in bearings fit, to the side members 36. Each telescoping rod 58 has a collar 6!, or other suitable stop member, on its upper portion and is surrounded by a helical spring 62 compressed between the collar 6! and a bearing portion 63 at its lower end so that the rod is resiliently biased in extended position to hold the arms in their extreme positions but may be resiliently compressed to permit a pivoting of the arms 48 from one position to the other.

In mounting a blanket on the cylindrical mandrel 2!, the arms 48 are swung to their upper positions shown in dotted line in Figure 4 and retained by the resilient telescoping rods 58. The mandrel 2! is mounted in the device by seating the trunnions 22 in the open bearings or sockets 3'! with the" clamping bar 21 adjacent the top position of the cylinder. The blanket is mounted on the mandrel 2!, the end portion !2 being registered therewith by inserting the pins 28 of the mandrel in the holes is of the blanket and then securing the end portion l2 by the clamp 2?.

The arms fit are then swung down so that the pressure roller engages the leading edge of the blanket adjacent the clamp 27, and the bolt 28 may be threaded to adjust the frictional tension 'between the brake shoes 46 and the drum d5.

mounted in a stretching device comprising a base member 35 having upstanding side members 36 Which are provided with U-shaped bearing brackets 3'! for removably receiving the shaft ends 22. The side members 36 also include sleeve bearings 38 which receive a cross shaft 39 that carries, and is fixed to, a pair of arms 4!! which are adapted to overlie the cylinder 2!.

These arms lie have bearings t! for receiving and carrying a cross-shaft a2 and terminate in longitudinal extending handles 43. The shaft 22 carries a pressure roller is for bearing on the blanket as the cylinder 2'! is rotated. The pressure roller M is frictionally restrained from rotation by abrake drum 85 carried by the shaft 22 at one end beyond one of the arms l!) as best shown in Fig. 5. A pair of arched brake shoes 46 straddlethe drum t5 and are pivotally mounted on the adjacent arm 36 by a pin 3?. The tension of the brake shoes 43 is regulated by a bolt 48 extending freely through a bore 4-9 in one of the shoe members and threading into a bore 50 in the other of the shoe members, a spring 5! being coiled about the bolt M3 between its head and the free shoe member so that, depend ng upon the threading of the bolt relative to the bore 50 of the shoe member, the tension of the spring 5! will be varied to regulate the pressure between the shoes 46 and the frictional resistance applied to the brake drum e5. Obviously, the drum 25 and/or the brake shoes 46 may be provided with suitable brake linings such as leather or the like.

The arms til are connected to the shaft. 39 by a tapered pin 52 extending diametrically through pin holes 53 in the arms ti! and a pin hole 54 in the shaft 39. The pin holes'53 of the arms 20 are slightly larger than the pin 52 to allow a slight play therebetween and to permit slight angular adjustment between the arms so that the The cylinder 2! is then rotated while the pressure roller M bears against the blanket and irons roller 44 the blanket ispulled taut around the drum and its opposite end !2' may be clamped to the cross bar, 24, if it has not previously been clamped thereto. The one or more bolts 3! may then be threaded against the bearing surface 32, to space the bar 24 from the surface 32 and hold the blanket in taut position about the cylinder 2!. The arms 40 may then be raised and the cylindrical mandrel 2! removed from the bearing sockets 3! and placed in a suitable lathe or grinding machine to grind the rubber surface of rubber-like impression sheet outermost to place said rubber-like sheet under tension, and grinding said rubber-like sheet to an even thickness while cylindrically taut.

WALTER H. SMITH.

(References on following page) 7 8 REFERENCES CITED Number Name Date The following references are of record in the 1,897,148 Rowen 1933 file of t t t: 1,906,436 Thompson May 2, 1933 2,099,024 Lewis Nov. 16, 1937 UNITED STATES PATENTS 5 2,207,999 Foster July 16, 1940 Number Name Date 2,287,872 Foster June 30, 1942 1,370,709 Smith Mar. 8, 1921 2,320,513 Drummond June 1, 1943 1,548,052 Maranvill Aug. 4, 1925 2,409,759 Hosking Oct. 22, 1946 

